1. Field of Endeavor
The present invention relates to a shape memory polymer device and more particularly to a resistively heated shape memory polymer device.
2. State of Technology
Shape-memory materials have the useful ability of being formable into a primary shape, being reformable into a stable secondary shape, and then being controllably actuated to recover their primary shape. Both metal alloys and polymeric materials can have shape memory. In the case of metals, the shape-memory effect arises from thermally induced solid phase transformations in which the lattice structure of the atoms changes, resulting in macroscopic changes in modulus and dimensions. In the case of polymeric materials, the primary shape obtained after processing and fixed by physical structures or chemical crosslinking. The secondary shape is obtained by deforming the material while is an elastomeric state and that shape is fixed in one of several ways including cooling the polymer below a crystalline, liquid crystalline, or glass transition temperature; by inducing additional covalent or ionic crosslinking, etc. While in the secondary shape some or all of the polymer chains are perturbed from their equilibrium random walk conformation, having a certain degree of bulk orientation. The oriented chains have a certain potential energy, due to their decreased entropy, which provides the driving force for the shape recovery. However, they do not spontaneously recovery due to either kinetic effects (if below their lower Tg) or physical restraints (physical or chemical crosslinks). Actuation then occurs for the recovery to the primary shape by removing that restraint, e.g. heating the polymer above its glass transition or melting temperature, removing ionic or covalent crosslinks, etc.